An increasing need exists for detector and alarm systems for protecting homes, stores, schools and the like from intruders. Burglar alarms have been available for many years, but cost and inconvenient operating features make the systems beyond the range of feasibility for buildings other than substantial business establishments or expensive homes. High labor costs make it increasingly difficult to provide guards and patrol services, and even at high cost these services are not always effective.
One of the major factors entering into the cost of a conventional burglar alarm system is the cost of installation. For example, an alarm system based on the use of photoelectric cells and light sources as the detection means would require a light source and photocell for each window or door of a building, or at least for each exterior wall having one or more windows or doors. Each of these sources and photocells would require connection to house wiring for power, and each cell would require a connection to a control alarm box or some type of monitoring system. The same sort of wiring or installation job would be needed for a system based on use of various other electrical detectors such as conductive tape on windows, switches on doors, etc. In any of these cases, the wiring job alone might cost many hundreds or even thousands of dollars, aside from the cost of the components of the system.
When component and installation costs become the major consideration in system design, then classically the reliability of the system suffers. For burglar alarms, one particularly annoying factor in a low quality system is that of false alarms. If the detectors of the system have a reasonably low threshold, i.e., the system is reasonably sensitive as it must be to be effective, then it follows that routine changes such as supply voltage variations, aging, etc., can cause false alarms. This difficulty with false indications is particularly troublesome if battery operated units are used to avoid house wiring.
It is accordingly a principal feature of this invention to provide a low cost, easily installed protection system or intrusion detector and alarm system for buildings such as homes or small businesses. Another feature is providing a burglar alarm type system which may be installed without connection to house wiring. Also, a feature is providing a battery operated surveillance system of the type described which is sensitive yet relatively free from false alarms caused by low battery voltage and the like.